The CDN1 transponder Cal/Val site has been established on the mountains of west Crete under a multiple cross-over point of Sentinel-3A & 3B, Jason series and that of Sentinel-6/Jason-CS.
A prototype microwave range transponder, a microwave radiometer (Radiometrics MP3000A), two Global Navigation Satellite System (GNSS) stations (Leica GR10 with a Leica AR25.R4 antenna and Trimble NetRS with a Trimble Zephyr Geodetic Antenna), and two meteorological stations (Vaisala WXT520 and Vaisala WXT534) comprise the main instrumentation of the CDN1 Cal/Val site. Hybrid power supply systems (solar, wind, diesel generator and batteries) provide continuous, safe and stable power to these instruments.
Microwave transponders have been applied as alternative tool for satellite altimetry calibration. A microwave transponder is an electronic equipment which receives the pulsed radar signal transmitted by the altimeter, amplifies it and retransmits it back to the satellite, where it is there recorded. The flight time of the radar signal is measured on the satellite, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this transponder technique, compared to the conventional sea‐surface calibration, stands for the fact that no ocean dynamics errors are involved in this calibration. To process transponder data, atmospheric corrections are needed. At first, those are determined through dedicated GNSS data processing. Delays caused by the ionosphere and the troposphere (wet & dry) are estimated using the propagation properties of the GNSS signal, and then reductions are made for altimetry signals.
A water vapor radiometer (WVR) constitutes another independent, and alternative to GNSS data processing technique for the determination of tropospheric delays at the Cal/Val transponder site. Customarily, a WVR uses two microwave frequencies: one at ~22.2 GHz (sensitive to water vapor in the atmosphere) and one at ~30 GHz (sensitive to liquid water). By measuring the power of thermal radiation in the atmosphere, the respective signal delays arising from water vapor and liquid water can be determined. Thus, the wet troposphere delays of altimetry signals can be determined (Teke et al., 2011).
The CDN1 Cal/Val site instrumentation is fully compliant with the FRM4ALT requirement of putting into use redundant and complimentary observing systems. For example, two diverse and independent approaches are used to derive atmospheric delays during a satellite overpass: (a) microwave radiometer and (b) GNSS-derived delays. Independent GNSS processing is also applied for not only the estimation of atmospheric delays, but also for the determination of absolute coordinates of the Cal/Val site.
The CDN1 transponder Cal/Val site has been operational since September 2015. It has been established initially to support absolute calibration of the Sentinel-3 satellite altimeters. In order to cross-calibrate Sentinel-3 with other altimetry missions, the transponder is also activated for other European (i.e., Sentinel-6, CryoSat-2) and international missions (i.e., Jason-2 and Jason-3). The CDN1 Cal/Val site operators have reached an agreement with the agencies operating these missions: EUMETSAT and CNES, respectively.
Till 30-Nov-2021, 441 transponder calibrations had been taken place for Sentinel-3A Pass No.14 (75 calibrations), Sentinel-3B (45 calibrations), CryoSat-2 (51 calibrations) and Jason-3 Pass No.18 (234 calibrations). On 18-Dec-2020, the first Sentinel-6 transponder calibration was carried out at the CDN1 Cal/Val during its tandem phase with Jason-3. Out of these 441 calibrations, 411 were successful whereas 30 had been cancelled mainly because of extreme weather conditions that could have put the transponder’s safety at risk.
Another objective of the SeRAC project is to refurbish and upgrade the existing ESA sigma-0 transponder for calibrating now the backscatter coefficient of Sentinel-3 SRAL instruments. That old ESA sigma-0 transponder had been used in the past to calibrate the Envisat Radar Altimeter 2(RA-2) (https://doi.org/10.1016/j.asr.2012.12.014).
Fig.1: The sigma0 transponder of ESA in June 2019
Refurbishment has been carried out by the Radio Analog Micro Electronics (RAME, https://ramesrl.it), Rome, Italy and completed in 2019. Final, Factory Acceptance Tests had been carried out in May 2019 in RAME, Rome, Italy.
Eight (8) field tests of the refurbished sigma0 transponder have been carried out in Tuscany, Italy under a crossover of Sentinel-3A Pass No. 193 and Sentinel-3B Pass No. 214.
Fig.2: The location of the crossover of Sentinel-3A and Sentinel-3B for the field testing of the sigma0 transponder
of ESA in Tuscany, Italy. Test sites are indicated by SP1A and SP1B.
The first field test was held on 29-Jan-2020, during which the transponder was placed on truck and above the SP1A reference benchmark (Fig. 2 and 3).
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| Fig.3: The field testing of the sigma0 transponder of ESA in Tuscany, Italy in January 2020 over the SP1A test site. | |
A series of six Sentinel-3A and two Sentinel-3B follow-on field tests have been carried out in the period January – September 2020. The Sentinel-3A signal as recorded by the sigma-0 transponder in September 2020 is shown in Fig. 4.
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| Fig.4: The received power of Sentinel-3A as captured during the field testing in September 2020 (Credit: RAME). | |
Absolute calibration of satellite altimeters, during their lifetime, by external and independent facilities, is a prerequisite for a continuous, homogenous and reliable monitoring of the earth, its oceans and climate change. These Cal/Val facilities ensure that altimetry observations are free of errors and biases, uninterrupted, but also tied from one mission to the next in an objective and absolute sense. Altimetry system’s responses have to be, thus, continuously monitored and controlled for their quality, biases, errors, drifts, although relations among different missions have to be established on a common and reliable earth-center reference system, maintained over a long period of time.
Up to now, absolute calibration of satellite altimetry is mainly provided by permanent ground calibration and validation (Cal/Val) facilities. These are located either offshore or on land (using microwave transponders) exactly under (absolute direct) or adjacent to satellite’s ground track on nearby coasts, to ensure monitoring of uncontaminated satellite observations (absolute indirect). Relative calibration of satellite measurements is also performed using multi-mission crossover analysis between reference altimeters and other missions (relative direct).
